This disclosure pertains to microlithography (transfer of a pattern to a sensitive substrate), especially as performed using a charged particle beam. Microlithography is a key technology used in the fabrication of microelectronic devices such as integrated circuits, displays, and micromachines. More specifically, the disclosure pertains, inter alia, to charged-particle-beam (CPB) microlithography performed using a pattern-defining segmented reticle on which the pattern is divided into multiple subfields each defining a respective portion of the pattern, and to methods by which distortion of projected subfield images, as caused by reticle deformation, is corrected quickly, inexpensively, and with high accuracy.
Conventional methods and apparatus are described below in the context of using an electron beam as a representative charged particle beam.
With the relentless drive to progressively smaller feature sizes (now less than 0.10 xcexcm) the pattern-resolution limitations of optical microlithography have become a major limitation. To solve this problem, considerable effort currently is being expended to develop a practical xe2x80x9cnext generationxe2x80x9d microlithography technology. A major effort to such end involves using a charged particle beam (e.g., an electron beam) as the lithographic energy beam. Charged-particle-beam (CPB) microlithography is expected to produce substantially better pattern resolution for reasons similar to the reasons for which electron microscopy yields better image resolution than optical microscopy.
Current CPB microlithography technology does not yet embody a solution to the problem of projecting an entire pattern in one shot from the reticle to the substrate. Consequently, according to one conventional method, the pattern is divided into individual exposure units usually termed xe2x80x9csubfieldsxe2x80x9d each defining a respective portion of the overall pattern. The subfields are defined on a xe2x80x9csegmentedxe2x80x9d reticle and exposed in a prescribed order subfield-by-subfield. This exposure scheme is termed xe2x80x9cdivided-reticle pattern transfer,xe2x80x9d as described for example in U.S. Pat. No. 5,260,151 and Japan Kxc3x4kai (published) Patent Document No. Hei 8-186070. As can be surmised, the optical field of CPB optics required to transfer a single subfield is much smaller than otherwise would be required to transfer the entire pattern in one shot. During transfer of each subfield, the respective subfield image is formed on the substrate in a manner such that, when exposure is complete, the subfield images are xe2x80x9cstitchedxe2x80x9d together in a manner by which they collectively form the entire contiguous pattern on the substrate.
The subfields typically are arrayed on the reticle in rows and columns, wherein each row has a length substantially equal to the diameter of the optical field of the CPB optical system. During exposure of a row of subfields, the charged particle beam is deflected laterally as required to transfer the subfields in the row in sequential order. In progressing from one row to the next, the reticle and substrate typically are scanned mechanically in opposite lateral directions.
From the foregoing, it will be understood that conventional divided-reticle pattern transfer exhibits substantially lower xe2x80x9cthroughputxe2x80x9d (number of wafer substrates that can be processed lithographically per unit time) than optical microlithography in which an entire die can be exposed in one shot.
Two types of reticles generally are used in divided-reticle pattern transfer. The first type is termed a xe2x80x9cscattering-stencilxe2x80x9d reticle, and the second type is termed a xe2x80x9cscattering-membranexe2x80x9d reticle. In a scattering-stencil reticle, pattern elements are defined by respective apertures (through-holes) in a xe2x80x9cCPB-scatteringxe2x80x9d membrane (usually of silicon) having a thickness of approximately 1 to 5 xcexcm. In a scattering-membrane reticle, pattern elements are defined by a corresponding patterned layer of a highly CPB-scattering material formed on a thin, relatively non-scattering membrane.
Both types of reticles summarized above are produced by first fabricating a suitable xe2x80x9creticle blankxe2x80x9d (typically made from a silicon wafer) including a reticle membrane, and then forming the pattern on or in the membrane. The pattern normally is formed by electron-beam drawing followed by etching of the membrane to form a scattering-stencil reticle or of the layer of highly scattering material to form a scattering-membrane reticle. Forming the elements of the pattern in this manner on the reticle membrane can result in distortion and deformation of the respective pattern portions as defined in the subfields. Distortion and deformation also may arise when the reticle is mounted on a reticle stage of the CPB microlithography apparatus by electrostatic chucking or the like. Whenever a lithographic exposure is performed with a deformed reticle, the pattern image as projected from the reticle onto a lithographic substrate exhibits a corresponding deformation, which degrades the accuracy of pattern transfer (especially manifest as overlay errors or stitching errors). Accordingly, minimizing reticle deformation is important from the standpoint of obtaining the best possible pattern-transfer accuracy.
Methods have been proposed for measuring reticle deformation before using the reticle for microlithography. Subsequent lithographic exposure using the reticle is performed while correspondingly correcting the deformation. Corrections are made by, for example, adjusting the projection-optical system of the microlithography apparatus to make appropriate changes to image magnification, rotation, and position. The adjustments are made based on the measurement data obtained prior to commencing lithography.
In one conventional method, measurement marks are defined on the support struts of the reticle between the subfields. Before using the reticle for lithographic exposures, relative positions of the measurement marks are determined using an inspection device such as a coordinate-measurement device. Detected positional anomalies indicating reticle deformation are corrected.
In another conventional method, measurement marks are defined on the membrane portions of individual subfields of the reticle, as disclosed in Japan Kxc3x4kai Patent Document Nos. Hei 11-30850, 11-142121, and 2000-124114. The marks are illuminated using an electron beam of the microlithography apparatus. The relative positions or dimensions of the marks are measured, and positional or dimensional anomalies indicating reticle deformation are corrected.
In actual practice there are many diverse causes of reticle deformation. As a result, sufficient correction of reticle deformation usually cannot be obtained using the conventional corrective schemes summarized above. Also, the conventional deformation-correction methods summarized above require long reticle-inspection times in order to ascertain positional errors in all the subfields of the reticle. Consequently, inspection costs can be prohibitively high.
In view of the shortcomings of conventional methods as summarized above, the present invention provides, inter alia, lithographic-exposure methods in which reticle deformation is measured substantially more rapidly, more inexpensively, and with greater accuracy than conventionally.
A first aspect of the invention is set forth in the context of a microlithography method, performed using a microlithography apparatus, in which a device pattern to be transferred onto a sensitive substrate is defined on a reticle that is divided into multiple subfields each defining a respective portion of the pattern. The reticle is illuminated subfield-by-subfield with an illumination beam to produce a corresponding patterned beam carrying an aerial image of the illuminated region of the reticle. The aerial image carried by the patterned beam is projected and focused as a subfield image at a respective location on the sensitive substrate, and the subfield images on the substrate are stitched together to form the device pattern on the substrate. More specifically, in the context of such a microlithography method, a first aspect of the invention is directed to methods for correcting deformation of the reticle. In an embodiment of such a method, multiple position-measurement marks are defined on the reticle. Using a reticle-inspection device separate from the microlithography apparatus with which the reticle is to be used for making lithographic exposure, respective positional coordinates of at least some of the position-measurement marks on the reticle are detected so as to produce a first set of reticle-deformation data. The reticle is then mounted in the microlithography apparatus, and respective positional coordinates of at least some of the position-measurement marks on the reticle are detected so as to produce a second set of reticle-deformation data. While performing exposure of the pattern from the reticle to the substrate, one or more of the position and distortion of each subfield is corrected according to both the first and second sets of reticle-deformation data.
In such methods, exposure of the pattern can be performed using a charged-particle illumination beam and a charged-particle patterned beam.
Further with respect to these methods, the first set of reticle-deformation data can comprise a respective linear component and a respective non-linear component. In such an instance the second set of reticle-deformation data desirably comprises a respective linear component and a respective non-linear component, and one or more of the position and distortion of each subfield desirably is corrected according to the non-linear component of the first set of reticle-deformation data and the linear component of the second set of reticle-deformation data. These methods further can comprise the step of, for each subfield, calculating data regarding a respective rotational error and data regarding a respective orthogonality error from the first set of reticle-deformation data. For each subfield, data regarding a respective magnification error are calculated from the second set of reticle-deformation data. While performing exposure, one or more of the position and distortion of each subfield is corrected according to at least some of the respective calculated rotational error, orthogonality error, and magnification error.
In any of these methods, multiple reticles can be produced using an identical manufacturing process for all the reticles. In such an instance the first and second sets of reticle-deformation data can be obtained from one of the multiple reticles. The first and second sets of reticle-deformation data can be used to correct, when using another of the multiple reticles for making a lithographic exposure, one or more of the position and distortion of each subfield.
Other methods according to the invention are set forth in the context of a microlithography method, performed using a microlithography apparatus, in which a pattern is defined by a reticle segmented into subfields each defining a respective portion of the pattern, wherein the methods pertain to correcting deformation of the reticle. In an embodiment, first and second sets of position-measurement marks are defined on the reticle. Respective coordinates of the first set of position-measurement marks are obtained to provide a first set of deformation data. From the first set of deformation data, linear-correction parameters of the first set of deformation data are calculated. Linear components of the first set of deformation data are obtained, and non-linear components of the first set of deformation data are calculated. Respective coordinates of the second set of position-measurement marks are measured to provide a second set of deformation data. From the second set of deformation data, linear-correction parameters of the second set of deformation data are calculated. Linear components of the second set of deformation data are obtained, and subfield-position-coordinate data are obtained for the second set of deformation data. The subfield-position-coordinate data of second set of deformation data are entered into a subfield-position-coordinate table. Respective linear distortions of the subfields of the reticle are calculated from subfield-coordinate-measurement data obtained from the second set of position-measurement marks. Respective non-linear distortions of the subfields of the reticle are calculated from the non-linear components of the first set of deformation data. The calculated linear and non-linear distortion data are entered into the subfield-position-coordinate table, and exposure of the subfields of the reticle is performed based on corresponding recalled data from the subfield-position-coordinate table.
In the foregoing method the position-measurement marks of the first set desirably are located on support struts, and the position-measurement marks of the second set desirably are located in peripheral subfields of the reticle. The respective coordinates of the first set of position-measurement marks desirably are measured using a coordinate-measuring device that is separate from the microlithography apparatus with which the reticle will be used for making lithographic exposures. The linear-correction parameters of the first set of deformation data desirably are calculated by substituting the first set of deformation data into a matrix-conversion model and performing a least-squares fit, thereby yielding xe2x80x9cfirst linear-correction parameters.xe2x80x9d The conversion model desirably is a matrix equation in which rotational error (xcex8), orthogonality error (xcfx89), magnification errors (Sx and Sy), and shifts (Ox and Oy) of respective center positions of the subfields are respective variables. The linear components of the first set of deformation data desirably are obtained by substituting corresponding design-mandated data into a conversion model into which the linear correction parameters of the first set of deformation data have been substituted. The non-linear components of the first set of deformation data desirably are calculated by subtracting the respective linear components from the first set of deformation data. The respective coordinates of the second set of position-measurement marks desirably are measured using the microlithography apparatus with which the reticle is to be used for making a microlithographic exposure (desirably using a through-the-reticle detection system of the microlithography apparatus). The linear-correction parameters of the second set of deformation data desirably are calculated by substituting the second set of deformation data into a matrix-conversion model and performing a least-squares fit, thereby yielding xe2x80x9csecond linear-correction parameters.xe2x80x9d The linear components of subfield distortion desirably are obtained by substituting corresponding design-mandated data into a matrix-conversion model into which the linear-correction parameters of the second set of deformation data have been substituted. Similarly, the subfield-position-coordinate data desirably are obtained by substituting non-linear components of the first set of deformation data into a conversion model into which the second linear-correction parameters have been substituted. The converted non-linear components of the first set of deformation data and the subfield-position-coordinate data of second set of deformation data desirably are entered into a subfield-position-coordinate table in a memory of a controller of a microlithography apparatus with which the reticle is to be used for making a lithographic exposure.
Alternatively, the non-linear components of the first set of deformation data can be regarded as non-linear components of subfield distortion as measured in the microlithography apparatus, without conversion.
In the foregoing method embodiment, the linear distortions of subfields of the reticle, as calculated from subfield-coordinate-measurement data obtained from the first set of position-measurement marks, desirably include rotational error and orthogonality error of the subfields. The non-linear distortions of subfields of the reticle, as calculated from subfield-coordinate-measurement data obtained from the second set of measurement marks, desirably include magnification error of the subfields. The calculated linear and non-linear distortion data desirably are entered into a subfield-position-coordinate table in a memory in a controller of a microlithography apparatus with which the reticle is to be used for making a lithographic exposure.
In view of the foregoing, deformation of a reticle as used in a microlithography apparatus can be estimated as a sum of the linear component of reticle deformation (as measured in the microlithography apparatus) and a modified non-linear component of reticle deformation (as measured outside the microlithography apparatus). Data concerning reticle deformation are obtained by measuring respective coordinates of displacement marks on the reticle. To obtain the linear components of the data, respective mark coordinates are linearly transformed using a 2xc3x972 matrix having, for each mark, four matrix elements and two scalar shifts determined by a least-squares method fitted to the measured reticle-deformation data obtained inside and outside the microlithography apparatus. To obtain the non-linear components of reticle-deformation data obtained outside the microlithography apparatus, the respective linear components simply are subtracted from the measurement data. After obtaining the estimated reticle-deformation data (i.e., data concerning respective coordinate displacements for each mark that is measured), the corresponding distortion and/or coordinate shift for each subfield is calculated taking mark displacement around the subfields (by each matrix and scalar shift) to fit each subfield. Each matrix is converted to respective rotational error, orthogonality error, and magnification error.
Another aspect of the invention is directed to microlithography apparatus. An embodiment of such an apparatus comprises a reticle stage on which a reticle is mounted for making a lithographic exposure of a pattern, defined on the reticle, from the reticle to a sensitive substrate. (The reticle is segmented into multiple subfields each defining a respective portion of the pattern.) The apparatus includes an illumination-optical system situated upstream of the reticle stage and configured for illuminating each of the subfields on the reticle with an illumination beam. The apparatus also includes a projection-optical system situated downstream of the reticle stage and configured for projecting and focusing a patterned beam, formed by passage of the illumination beam through or from an illuminated subfield of the reticle, at a selected location on a surface of the sensitive substrate. The apparatus also includes a substrate stage situated downstream of the projection-optical system and configured for holding the sensitive substrate while a lithographic exposure is being made. The apparatus also includes means for detecting deformation of the reticle mounted to the reticle stage. The apparatus also includes a controller connected to and configured for controlling operation of the reticle stage, the illumination-optical system, the projection-optical system, the substrate stage, and said means for detecting reticle deformation, so as to achieve lithographic transfer of the pattern from the subfields of the reticle to corresponding locations on the sensitive substrate. The controller desirably comprises a first memory configured for storing a first set of reticle-deformation data detected using a reticle-inspection device separate from the microlithography apparatus, a second memory configured for storing a second set of reticle-deformation data detected by the microlithography apparatus, and a correction calculator configured for calculating a position and/or deformation of each subfield as required from the first and second sets of reticle-deformation data recalled from the first and second memories, respectively, and for calculating respective corrections to be applied as each subfield is being exposed lithographically.
The correction calculator further can comprise an exposure-position calculator configured to recall data from the first and second memories, and to calculate, based on the recalled data, respective corrections of exposure position required at various locations on the reticle. The correction calculator further can comprise a third memory for storing a subfield-coordinate-position table in which data produced by the exposure-position calculator are stored. The correction calculator further can comprise a command generator configured to recall data from the subfield-coordinate-position table and issue appropriate control commands to the projection-optical system based on the recalled data.
The highest correction accuracy can be obtained by measuring the coordinates of respective marks distributed on the surface of the reticle so that current deformation data for the reticle can be obtained, and by correcting the respective positions or dimensions of each subfield on the basis of this data. In conventional methods, measurement of the respective coordinates of the marks on the reticle using the microlithography apparatus requires substantial time, which can adversely affect the throughput of the microlithography apparatus. According to various methods as summarized above, measurement time inside the microlithography apparatus is shortened by obtaining certain measurements of the respective coordinates of respective marks using an inspection device separate from the microlithography apparatus, and measuring the coordinates of only some of the marks on the reticle using the microlithography apparatus, thereby avoiding decreases in throughput. Also, since the respective position of each subfield is corrected using deformation data obtained by the separate inspection device and deformation data obtained by the microlithography apparatus, reticle deformation is corrected with high accuracy.
Further with respect to the foregoing methods, it is desirable that the first set of deformation data be divided into a respective linear component and a respective non-linear component. Similarly, it is desirable that the second set of deformation data be divided into a respective linear component and a respective non-linear component. Exposures are performed while the position and/or deformation of each subfield are corrected using the non-linear component of the first set of deformation data and the linear component of the second set of deformation data 2.
Also, since exposure corrections are performed on the basis of deformation data obtained by the separate inspection device and deformation data obtained by the microlithography apparatus, reticle deformation is corrected quickly and with high accuracy.
Situations can arise in which the thermal environment is different in the separate inspection device and in the microlithography apparatus. Accordingly, taking thermal expansion of the reticle and other factors into account, exposure-correction accuracy and precision are improved by measuring parameters such as magnification errors inside the microlithography apparatus. Double corrections are avoided by measuring linear components of deformation inside the microlithography apparatus and non-linear components of deformation using the separate inspection device.
If multiple reticles are manufactured using the same manufacturing process, then reticle-deformation data can be obtained from a single reticle among the multiple reticles. Deformation data from the one reticle can be used for correcting exposure performed with other reticles made by the same process. Because reticles manufactured by the same manufacturing process exhibit reproducibility with respect to reticle deformation, reticle inspection time can be shortened and inspection costs reduced by inspecting only one reticle among the multiple reticles, and using the obtained correction data for correcting exposure using the other reticles.
The foregoing and additional features and advantages of the invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.